1. Field of the Invention
The present invention specifically relates to a transmission apparatus using a polar modulation scheme.
2. Description of the Related Art
In the design of conventional transmission modulation apparatuses, there is generally a trade-off between efficiency and linearity. Recently, however, a technology has been proposed that enables compatibility between efficiency and linearity to be achieved in a transmission modulation apparatus by using polar modulation.
FIG. 1 is a block diagram showing a sample configuration of a transmission modulation apparatus employing polar modulation.
Polar coordinate change section 11 divides a baseband modulation signal into baseband amplitude modulation signal S1 representing the amplitude component (for example, √I2+Q2) and baseband phase modulation signal S2 representing the phase component (for example, the angle formed by the modulation symbol and the I-axis).
Power supply voltage change section 12 changes power supply voltage S3 supplied from a power supply (not shown) into power supply voltage S4 and outputs changed power supply voltage 4 to regulator 14.
Power apply section 13 forms amplitude modulation signal S6 by multiplying baseband amplitude modulation signal S1 with control signal S5.
Regulator 14 (changes power supply voltage S4 using amplitude modulation signal S6. Further, regulator 14 supplies power supply voltage S8, which is obtained by changing power supply voltage S4 using amplitude modulation signal S6, to a power supply input terminal of power amplification section 16.
Phase modulation section 15 changes a high-frequency signal into phase modulation high-frequency signal S7 by phase modulation using baseband phase modulation signal S2.
Power amplifier 16 amplifies phase modulation high-frequency signal S7 using power supply voltage S8 as a power supply and outputs RF transmission signal S9, which is obtained by amplifying the power of phase modulation high-frequency signal S7, to isolator 17.
Isolator 17 allows radio waves to pass in a certain direction without attenuation but absorbs the power of the radio waves in the opposite direction. By this means, it is possible to prevent a signal received at antenna 18 from entering power amplifier 16. Further, since isolator 17 is provided, even if the load of antenna 18 fluctuates, the negative influence does not extend to power amplifier 16, so that the operation of power amplifier 16 does not become unstable. Further, the S/N ratio of the output signal of power amplifier 16 is prevented from being deteriorated. Thus, isolator 17 prevents a reflected signal from antenna 18 from being conveyed to power amplifier 16 and has no influence to power amplifier 16 according to the load fluctuation of antenna 18.
Antenna 18 transmits RF transmission signal 10.
When transmission modulation apparatus 10 employing the above-described configuration is used for terminals such as mobile phones, isolator 17 is provided so as to stabilize the characteristics of power amplifier 16. On the other hand, the insertion of isolator 17 causes power loss and results in an increase of the area occupied by transmission modulation apparatus 10.
Although removing isolator 17 from transmission modulation apparatus 10 would solve the problem of power loss and the problem of increased area, the characteristics of power amplifier 16 becomes unstable. That is, by removing isolator 17 from transmission modulation apparatus 10, the load of power amplifier 16 is prone to fluctuation.
To be more specific, in the case of mobile terminals such as mobile phones, the use environment changes variously, and, accompanying this, the load of power amplifier 16 (the load of antenna 18) fluctuates sensitively. For example, during a call, when the case of the device is close to the human body or when a mobile phone is used near a metal desk, the antenna of the mobile phone is capacitive-coupled with the human body or a metal board, and the load of the transmission modulation apparatus fluctuates significantly.
This load fluctuation increases the current value ICC of the current flowing from power supply voltage change section 12 to power amplifier 16, and decreases the output power of power amplifier 16. As a result, there is a problem that the power loss of power amplifier 16 is increased as shown in equation 1 and the temperature of power amplifier 16 significantly increases by heat converted from the power loss of power amplifier 16.
[1]Power loss=supply power(VccICC)−output power(Pout)  (Equation 1)
As described above, there is a problem that load fluctuation makes the power loss of a power amplifier increase and the increased power loss makes the amount of heat produced in the power amplifier increase. Therefore, in a mobile phone mounting the above-described transmission modulation apparatus, the temperature of the case increases to high temperature and its life and characteristics degrade. In particular, in a mobile phone touching the human body upon use, it is difficult to use the mobile phone when the surface temperature of the mobile phone main body becomes high. Therefore, a problem of an increase of the amount of heat produced in the power amplifier is posed.
To solve these problems, Patent Document 1 discloses a method of estimating load fluctuation by detecting the current of a power amplifier, and controlling to reduce load fluctuation of variable load in a power amplifier output terminal. FIG. 2 illustrates a configuration of main components of the polar modulation transmission apparatus disclosed in Patent Document 1.
As disclosed in Patent Document 1, to reduce negative influence caused by the load fluctuation of a power amplifier, a circuit for estimating the load fluctuation of the power amplifier and a circuit for controlling the load may be provided in the polar modulation transmission apparatus.
However, the scale of a circuit for estimating load fluctuation or controlling the load becomes larger. Therefore, when the circuit is applied to terminals such as mobile phones, it is hard to make these terminals smaller.    Patent Document 1: Japanese Patent Application Laid-Open No. 2000-295055.